The present invention is directed to an improved process for the production of phosphoric acid by the wet process. The invention is directed to the production of phosphoric acid by the calcium sulfate hemihydrate or simply the hemihydrate process. The present invention is directed to a process in which organic sulfonate reagents are used to increase the crystal size of the calcium sulfate hemihydrate crystals formed during the production of phosphoric acid.
The hemihydrate wet process for the production of phosphoric acid is one of the processes used in the industry today. Other processes used are the gypsum, or the dihydrate process and the anhydrite process. All three prcesses are named from the by-product calcium sulfate produced during the production of phosphoric acid. The gypsum or dihydrate process is run at a temperature of 90.degree. C. or less and a P.sub.2 O.sub.5 concentration of about 30% in the liquid portion of the slurry. Increasing the temperature to about 90.degree. to 120.degree. C. and the P.sub.2 O.sub.5 concentration from about 30 to 45% in the liquid phase will yield calcium sulfate hemihydrate. If, however, one chooses to run the phosphoric acid wet process at 130.degree. C. and a P.sub.2 O.sub.5 concentration greater than 30% than one obtains calcium sulfate anhydrite as the by-product. Advantages and disadvantages of each process are described in "Phosphoric Acid" Part One, edited by A. V. Slack, Marcel Dekker, Inc., New York, N.Y., 1968.
Phosphate rock and sulfuric acid are reacted with a slurry comprising phosphoric acid, sulfuric acid, monocalcium phosphate, and calcium sulfate hemihydrate. The temperatures and P.sub.2 O.sub.5 concentrations are such that the main product from the reaction of the phosphate rock and the sulfuric acid will be calcium sulfate hemihydrate and phosphoric acid. Little, if any, calcium sulfate dihydrate will be observed in said reaction. The slurry so produced is then sent to a recovery section where the solids are separated from the liquid. This is usually done by means of a filter. The rate of filtration or the filterability of the slurry will be dependent among other things upon the size of the calcium sulfate hemihydrate crystals produced during the reaction.
The literature makes reference to the use of organic sulfonic acids and derivatives thereof as crystal growth modifiers to be used in wet process phosphoric acid processes, as agglomerating agents for use in wet process phosphoric acid slurries and as a reagent to moderate the conversion of calcium sulfate hemihydrate to calcium sulfate dihydrate or the reverse, namely the conversion of calcium sulfate dihydrate to calcium sulfate hemihydrate.
Slack (reference cited page 279) describes the use of a surface active agent in the dihydrate process, said surface active agent is used to promote the growth of small crystals. It appears as if the surface active agent in the dihydrate wet process acts as a nucleation poison to gypsum and reduces the number of gypsum nuclei formed, thereby favoring growth of large crystals and making the operation more difficult to upset by changes in conditions. The formation of larger crystals makes it easier to filter the slurry so formed. D. W. Leyshon et al, U.S. Pat. No. 3,192,014 describes a process for the preparation of phosphoric acid by means of the dihydrate wet process in which an additive selected from the group consisting of alkylbenzenesulfonic acids having an alkyl group from 9 to 12 carbon atoms, isopropylnapthalene sulfonic acid and the alkali metal salts of said acids are used to improve the filterability of the slurry produced. The additive is used from about 0.1 to about 3.2 pounds per ton of phosphate rock.
Slack (reference cited, page 383) describes the use of surface active agents in the hemihydrate wet process. Tests have shown that the formation of small unstable hemihydrate crystals as observed in previous processes, does not occur when a surface active agent or surfactant is added to the system. The surface active agent used was an alkylbenzenesulfonic acid.
A. F. Sirianni et al, U.S. Pat. No. 3,796,790 describes a process for the recovery of concentrated phosphoric acid from a suspension of gelatinous or finely divided precipitate such as the calcium sulfate in the process for producing phosphoric acid. The process involves treating the suspension with a particular surface active agent and a particular bridging liquid, said bridging liquid is a liquid hydrocarbon such as naphtha, kerosene, fuel oil, low viscosity processed oils, gas oils and petroleum aliphatic solvents. Agglomerates are formed when the suspension is vigorously mixed with the surface active agent and the liquid hydrocarbon. Said agglomerates are then removed by decanting, filtering, or centrifuging.
Several patents describe the recrystallization of calcium sulfate from one hydrate form to another. The patent issued to H. Akazawa et al, U.S. Pat. No. 3,645,677 is representative. Calcium sulfate hemihydrate is hydrated to calcium sulfate dihydrate in the presence of at least one surface active agent, selected from the group consisting of alkylaryl sulfonic acids, alkylnapthalene sulfonic acid, sulfuric acid esters of higher alcohols and salts thereof, and a mixed acid comprised of sulfuric acid and phosphoric acid from the ratio of about 0.4 or greater. Large, easily filterable purified calcium sulfate dihydrate crystals are prepared.
Other types of additives have been employed which aid in the growth of calcium sulfate crystals. Hey et al, U.S. Pat. No. 3,653,727, uses a mixture of an amide and a fatty acid as an anti-foam agent. The agent also improves the rate of calcium sulfate filtration.